1. Field of the Invention
This invention relates to a method for the operation of a flash smelting furnace used for producing from copper or nickel sulfide ore the matte as a smelting intermediate for the corresponding metal, which method is particularly aimed at enhancing the ability of the furnace to accomplish the treatment.
2. Description of the Prior Art
A flash smelting furnace which uses sulfide concentrates as a raw material and which is called a "flash furnace" enjoys many advantages as compared with smelting furnaces of other types, and yet suffers from many disadvantages. For the sake of illustration, a conventional flash furnace for copper will be described with reference to FIG. 2.
In a flash smelting furnace 1, powdered concentrate 2 and reaction gas 3 such as preheated air are jointly blown into a reaction shaft 5 of the furnace through a concentrate burner 4 at the top of the furnace. Inside the reaction shaft 5, sulfur and iron which are combustible components of the powdered concentrate 2 react with the hot reaction gas 3 and melt themselves. The resulting melt is allowed to collect in a settler 6. In the settler, which serves as a reservoir for the melt, the melt is divided by virtue of differences in specific gravity into a matte 7 which is a mixture of Cu.sub.2 S and FeS and a slag 8 which consists mainly of 2FeO.SiO.sub.2. The slag 8 is released through a slag discharge outlet 9 and introduced into an electric slag cleaning furnace 10. In the meantime, the matte 7 is tapped through a matte discharge outlet 11 in compliance with the demand from a converter which constitutes itself a next step of operation.
A hot waste gas 12 emanating meanwhile from the flash smelting furnace 1 is passed through the settler 6 and an uptake 13 and cooled in a boiler 14. The slag which has entered the electric slag smelting furnace 10 is kept heated with the heat generated by the heat fed in through electrodes 15 and, when necessary, mixed as with lumps of ore and flux introduced into the electric slag cleaning furnace 10, with the result that the copper component is further allowed to settle to the bottom of the furnace and the slag containing a barely remaining copper component is only released out of the system via an outlet 16.
The conventional flash smelting furnace has entailed many drawbacks as indicated below.
(1) Inside the reaction shaft 5, supplemental fuel is used to make up for insufficient calorific supply. Owing to the heat of reaction of the concentrate as the raw material and the heat of combustion of the supplemental fuel, the temperature of the atmosphere inside this reaction shaft 5 is elevated to a fairly high level. An attempt at increasing the amount of the concentrate to be treated results in an accelerated wear of the refractory bricks lining the reaction shaft 5 by fusion. Thus, amount of the concentrate to be forwarded through the concentrate burner 4 and treated per unit time is inevitably limited to an extent at which the wear of the bricks by fusion is tolerable. This wear of the bricks by fusion bears closely upon the thermal load of the reaction shaft. The wear occurs conspicuously if the thermal load exceeds 350,000 Kcal/m.sup.3.hr. Thus, the thermal load is desired to be not more than 250,000 Kcal/m.sup.3.hr.
An addition to the amount of treatment is realized by increasing the inside diameter and height of the reaction shaft. Since this dimensional increase inevitably entails an increase in the surface area of the reaction shaft, the amount of heat radiated is proportionately increased and, to make up for this loss of heat, the amount of supplemental fuel to be used is increased. Further, such an exclusive increase in the reaction shaft as considered here is fairly difficult to realize in an existing flash furnace.
As a means of permitting treatment of an increased amount of concentrate, a method which resorts to an increase in the oxygen content of the preheated air 3 or an increase in the degree of oxygen enrichment is conceivable. Again in this case, the atmosphere in the reaction shaft 5 suffers further elevation of temperature. From the standpoint of avoiding the loss of the lining refractory bricks by melting, therefore, the amount of concentrate to be treated has its own upper limit.
(2) In the concentrate burner 4, the powdery concentrate 2 and the reaction gas 3 are blown into the space of the reaction shaft 5. The melt consequently formed therein falls dropwise into the settler 6, where it is separated into the matte and the slag. The hot waste gas 12 from the flash furnace 1, therefore, contains a large amount of dust. This dust tends to accumulate in the uptake 13, in the part interconnecting the uptake 13 and a boiler 14, and inside the boiler 14, and forms an obstacle to the passage of gas.
Since this dust contains valuable metals, it is recovered at the boiler and an electrostatic precipitator and returned to the flash furnace 1 as entrained by the concentrate 2 being fed thereto. This dust, however, is in an oxidized or sulfated state because it has undergone an oxidation reaction in the atmosphere containing SO.sub.2. When the dust is recycled in the reaction shaft 5, the amount of supplemental fuel required is increased and, moreover, the ignition and combustion of the concentrate is impeded by the absorption of heat due to the decomposition of the sulfate components, with the inevitable result that the portion of the concentrate escaping the combustion induces an increase in the amount of scattered dust and an increase in the amount of unmelted concentrate on the bath surface. This contradictory relation resembles what occurs in a powdery fire extinguisher which kills fire by the heat of its own decomposition. Further, the incombustible dust which has undergone a further oxidation reaction has such a high melting point that a large proportion thereof will be taken out of the furnace as entrained by the waste gas, giving rise to a vicious cycle of increasing the amount of dust to be produced.
Such incombustible raw materials as powdered residual copper which has an extremely low sulfur content is also treated in the reaction shaft 5. This treatment has the same problem as the treatment of the recovered dust.
(3) An attempt at increasing the amount of the concentrate to be treated in the concentrate burner 4 results in an increase in the ratio of dust generation described in (2) above because the space density and distribution of the concentrate and the flow rate of gas within the reaction shaft 5 are suffered to deviate from the optimum reaction conditions. From the standpoint of the ratio of dust generation, therefore, the amount of the concentrate to be forwarded through the concentrate burner 4 and treated has its own upper limit.
(4) The inside of the reaction shaft 5 has an oxidizing atmosphere. Particularly the low-temperature zone in which the powdery raw material blown in through the concentrate burner 4 has not yet attained sufficient temperature elevation is liable to form magnetite. This magnetite throws many hindrances in the way of furnace operation. For example, the magnetite increases the viscosity of the slag to the extent of impairing separation of the slag from the matte and increasing the copper content of the slag. Further since the magnetic has a high density, it settles and accumulates on the hearth and raises the surface level of the hearth and decreases the available furnace internal volume. Moreover, the magnetite combines itself with other oxides such as Cr.sub.2 O.sub.3 and gives rise to a high viscosity slag in the intermediate layer between the matte and the slag and, consequently, interferes with the separation between the matte and the slag. This high viscosity slag has a high melting point. The high melting point coupled with the high viscosity renders the release of the slag through the slag discharge outlet difficult.